Several studies to date have demonstrated abnormalities in brain structure in adolescent and emerging-adult cannabis users. Cognitive deficits, including slowed processing speed, reduced verbal memory, sustained attention and sequencing ability, were measured following a month of monitored abstinence in one study, although another found significant recovery following 4 months of abstinence. This may be partially due to a relatively small sample size (19 per group) and the authors did not report specific effect sizes obtained when comparing the cannabis users with controls. Takagi and colleagues did not find differences in cognitive inhibition on a computerized task between inhalant users, cannabis users, and control adolescents. ![]() With one exception, several cross-sectional studies in cannabis-using youth without psychiatric comorbidities report cannabis-related cognitive deficits including reduced processing speed, complex attention, verbal memory, executive functioning, and risky sexual behavior. After controlling for gender, nicotine use, comorbid schizophrenia, and alcohol use, they also found specific deficits in executive functioning, sustained attention, verbal list learning, and psychomotor speed associated with persistent cannabis dependence, findings that are generally consistent with cross-sectional studies. ![]() Specifically, individuals who never regularly used cannabis had a slight increase (0.8 IQ point) in IQ from childhood into adulthood, while those diagnosed with cannabis dependence on at least three or more study occasions had an average loss of 5.8 IQ points. In the largest prospective, longitudinal study to date, Meier and colleagues followed a sample of 1,037 from birth to age 38 and found that 153 participants met criteria for cannabis-use disorders (CUD) at least once during the follow-up and individuals with more persistent cannabis use demonstrated the greatest reduction in IQ. ![]() Two longitudinal studies that followed adolescents with substance use disorders over 8 years found that increased cannabis use during the follow-up period significantly predicted poorer attention and verbal memory. Although controversy exists in the adult literature, evidence is building to suggest that regular cannabis use during the teenage or emerging adult years (typically ages 15–25 years) is associated with cognitive deficits.
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